Oxyhydroxide, of iron

Table l.i Oxides, hydroxides and oxyhydroxides of iron. Reprinted with permission from Jambor and Dutrizac, 1998. Copyright (1998) American Chemical Society. 

Copper acetate (0.125moll-1) has been used to displace metals sorbed on organic matter and on oxyhydroxides of iron (Soon and Bates, 1982), while 0.05moll-1 lead nitrate released specifically bound copper (Miller et al., 1986a, b). 

The oxyhydroxides of iron consist essentially of close packed layers of oxygen atoms with iron atoms situated in the interstitial holes. Thus the iron atoms, depending on whether they are surrounded by 6 or 4 oxygen atoms have either octahedral or tetrahedral coordination. A crystalline model of the ferritin core has been proposed by Harrison et al. (85) which fits the X-ray and electron diffraction data involving close packed oxygen layers with iron randomly distributed among the eight tetrahedral and four octahedral sites in the unit cell. 

Solids, that precipitate in the upper levels in Lake Vanda and then sink through the water column, encounter increasing temperatures (i.e.. Fig. 19.32) and acidity expressed as the pH in Fig. 19.36. As a result, solid particles that form in the upper levels of the lake, such as the oxyhydroxides of iron and manganese, may dissolve at deeper levels. Crystalline as well as amorphous... 

In weathering, steel rust formed on atmospheric corrosion in different environments is composed of crystalline compounds like haematite, magnetite and oxyhydroxides of iron like goethite, akaganeite, lepidocrocite and feroxyhite apart from amorphous ferric oxyhydroxide rust. These rust constituents transform to one another during wet-dry cycles of atmospheric exposure [17].Various phases of corrosion products formed in progressive exposure to atmosphere are given in Table 1.1 [6, 18]. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Shannon R.D., White J.R. The selectivity of a sequential extraction procedure for the determination of iron oxyhydroxides and iron sulfides in lake sediments. Biogeochem 1991 14 193-208. 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

Thin films of a composite nickel-iron (9 1 Ni/Fe ratio) and iron-free oxyhydroxides were deposited from metal nitrate solutions onto Ni foils by electroprecipitation at constant current density. A comparison of the cyclic voltammetry of such films in 1M KOH at room temperature (see Fig. 6) shows that the incorporation of iron in the lattice shifts the potentials associated formally with the Ni00H/Ni(0H)2 redox processes towards negative potentials, and decreases considerably the onset potential for oxygen evolution. The oxidation peak, as shown in the voltammo-gram, is much larger than the reduction counterpart, providing evidence that within the time scale of the cyclic voltammetry, a fraction of the nickel sites remains in the oxidized state at potentials more negative than the reduction peak. 

As we pointed out earlier, the H subunit catalyses the ferroxidase reaction, which occurs at all levels of iron loading, but decreases with increasing amounts of iron added (48-800 Fe/ protein). Reaction (19.8) catalysed by both FI- and L-chain ferritins, occurs largely at intermediate iron loadings of 100-500 Fe/protein. Once nucleation has taken place, the role of the protein is to maintain the growing ferrihydrite core within the confines of the protein shell, thus maintaining the insoluble ferric oxyhydroxide in a water-soluble form. 

Type 3. In this category dissociation of the water molecule is only partial and surface OH species dominate. Cobalt is an example (67). Iron appears (6S) to form a mixed oxyhydroxide adlayer analogous to FeO OH. This renders the surface unreactive to further attack by 02(g), even though it is estimated to be no more than a single monolayer. Oxidation of iron by 02(g) is well known to be multilayer at 295 K. Similar data have also been reported by Gimzewski et al. 69) with iron. 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

Amouric, M. Baronnet, A. Nahon, D. Didier, P. (1986) Electron microscopic investigations of iron oxyhydroxides and accompanying phases in lateritic iron-crust pisolites. Clays Clay Min. 34 45-52... 

Bauer, Ph. Genin, J.M. Rezel, D. (1986) Mossbauer effect evidence of chlorine environments in ferric oxyhydroxides from iron corrosion in chlorinated aqueous solution. Hyperfme Interactions 28 757-760... 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Misawa,T Hashimoto, K. Shimodaira, S. (1974) The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature. Corrosion Sci. 14 131 — 149... 

Chem. Soc. Faraday Trans. I. 71 1623-1630 Rustad, J.R. Felmy A.R. Hay, B.P. (1996) Molecular statics calculations for iron oxide and oxyhydroxide minerals Toward a flexible model of the reactive mineral-water interface. Geochim. Cosmochim. Acta 60 1553—1562 Ryan, J.N. Gschwend, P.M. (1991) Extraction of iron oxides from sediments using reductive dissolution by titanium(III). Clays Clay Min. 39 509-518... 

Shah Singh, S. Kodama, H. (1994) Effect of the presence of aluminum ions in iron solutions on the formation of iron oxyhydroxides (FeOOH) at room temperature under acidic environment. Clays Clay Min. 42 606—613... 

Sidhu, P.S. Gilkes, R.J. Posner, A.M. (1981a) Oxidation and ejection of nickel and zinc from natural and synthetic magnetites. Soil Sci. Soc. Am. J. 45 641-644 Sidhu, P.S. Gilkes, R.J. Cornell, R.M. Posner, A.M. Quirk, J.P. (1981) Dissolution of iron oxides and oxyhydroxides in hydrochloric and perchloric acids. Clays Clay Min. 29 269-276... 

The challenge with parenteral iron therapy is that parenteral administration of inorganic free ferric iron produces serious dose-dependent toxicity, which severely limits the dose of that can be administered. However, when the ferric iron is formulated as a colloid containing particles with a core of iron oxyhydroxide surrounded by a core of carbohydrate, bioactive iron is released slowly from the stable colloid particles. In the USA, the three available forms of parenteral iron are iron dextran, sodium ferric gluconate complex, and iron sucrose. 

Elemental iron, iron-bearing oxyhydroxides and iron-bearing aluminosilicate minerals have been observed to promote the reduction and precipitation of Cr(VI). The overall reactions for the reduction of Cr(Vl) by Fe° and the subsequent precipitation of Cr(III) and Fe(lll) oxyhydroxides are ... 

During the first six hours of extraction, more iron is removed from the altered granite than from the fresh granite. The extra iron from the altered granite is believed to be that associated with the production of iron oxyhydroxides during the 101-d exposure of the granite to GGW. 

Sorption of Co onto oxyhydroxides precipitated in stream beds has been shown to be virtually irreversible (9). Diffusion into the crystal lattice of iron-bear gg minerals has also been suggested (10) The slow removal of Co during the second 48-h KTOX extraction is consistent with this mechanism. 

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